"In the UK we expect less rain in summer and more in winter, but inevitably we may see periods of several dry years in a row."

Heavy downpours in parts of the country over Christmas have done little to improve the situation because the water is unable to soak into the dry ground.

Baroness Young, chief executive of the Environment Agency, warned: "If it does not rain above average between now and the spring there won't be enough in the groundwater and reservoirs to keep us going."

Bewl Water in Kent, the largest area of open water in South East England, is only 55 per cent full despite recent rain. The UK's largest reservoir, in Kielder, Northumberland, is only 70 per cent full compared with a winter average of 80 per cent.

Steve Hodgson, spokesman for Severn Water, said: "If we get some sensible rainfall over the winter the reservoirs will look very different. But if it is a very dry winter it will look serious."

Anglia Water said it believed it could manage with "slightly less than average rainfall this winter".

But a spokesman added: "If we get much less than average it will be much more serious."

The Environment Agency is taking no chances and is advising the public to limit water use for the foreseeable future.